In an attach procedure of mobile communications, user equipment (UE) sends an attach request message to an mobility management entity (MME) by using an evolved NodeB (eNB). The attach request message carries UE capability information such as a network capability and a security capability. The MME provides a service for the UE according to the received UE capability. When the attach request message has no integrity protection, for example, the attach request message has no integrity protection in a scenario in which the UE registers with a network for the first time, if an attacker implements a man-in-the-middle attack, to modify the UE capability information sent by the UE to the MME, the MME provides a service for the UE based on the modified UE capability information. Consequently, the UE possibly cannot use some services. For example, the attacker removes voice domain preference and UE's usage setting in the UE capability information, and adds an additional update type-SMS only parameter. As a result, the UE can use only an SMS message service, and cannot use a voice call service.